Nitrogen cycling and marine redox evolution during the Ediacaran–Cambrian transition

The Cambrian Explosion was an unprecedented bioevolutionary event that witnessed rapid diversification of marine invertebrate phyla and establishment of metazoan-dominated marine ecosystems. Nitrogen is a critical nutrient element essential for all life on Earth, and its biogeochemical cycling in the ocean is tightly associated with marine redox conditions. Numerous nitrogen isotope investigations of the Ediacaran-Cambrian (E-C) transition have been undertaken, but an integrated analysis of contemporaneous nitrogen cycling has not been achieved yet. Here, we compile published nitrogen isotope data for the South China Craton over the interval from ∼550 Ma (late Ediacaran) to ∼514 Ma (late Stage 3 of early Cambrian) with the goals of identifying key changes in the nitrogen cycle and their relationship to marine redox evolution and the Cambrian Explosion. Combined with independent redox proxy data, our δ15N dataset provides insights into spatio-temporal variation in rates of denitrification and N2 fixation induced by marine redox fluctuations, which constrains the distribution of suboxic environments and the relative position of the oceanic redoxcline. On this basis, we propose a new model of marine nitrogen-cycle evolution during the E–C transition in which (1) nitrate availability modulated the ecological development and distribution of eukaryotic primary producers, and (2) nitrate-replete ecological niches for eukaryotic primary producers, especially benthic algae expanded significantly during Cambrian Age 3, and (3) increasing biological pump efficiency promoted organic burial and net O2 release, thus contributing to oceanic oxygenation and the radiation of early animals.